This invention relates to glass compositions especially biocompatible glass useful for encapsulation.
A method for obtaining information from animals or via animals comprises implanting a mini-transmitter of transponder in the animal body. Transponders essentially comprise a transmitter antenna and a memory chip and/or sensor or another component and allow the code or other information stored in the memory chip to be read out at any time via an external receiver. Systems of this type are commercially available as animal information and identification systems. In this way, for example, the animal can be identified and its location, body temperature and other data determined and used for scientific purposes, for example to determine migratory routes or to control automatic feeding systems. The transponders, which are extremely sensitive to chemical attack, are screened from body fluids by encapsulation. Known encapsulation materials are some plastics which are expensive or difficult to machine, and glass. Since the encapsulation may, for example in the labelling of wild animals, have to remain in the body for life, i.e., for up to 30 years, glass is a particularly suitable encapsulation material due to its long-term stability, low price and good processing properties. The encapsulation is carried out by introducing the transponder into a glass tube sealed at one end, and then sealing the other end by melting.
The principal disadvantage of the glasses used hitherto for this purpose is that the melt-sealing can only be carried out relative inconveniently. Due to the sensitivity of the transponder to corrosion, it is necessary not only to encapsulate it hermetically, but also to operate it in the absence of traces of corrosive gases, i.e., in a protective-gas atmosphere or in a vacuum. It is not possible to melt the glass using a flame since the water produced by combustion corrodes the transponder in the long term.
The glasses used hitherto are therefore usually sealed by the heat of resistance-heated elements, in general by coil melting. In coil melting, the heat necessary for the melting is generated by an electrical resistance-heated incandescent wire coil. The heat transfer from the coil to the glass takes place principally by convection. However, this has the disadvantage that the melting takes a relatively long time, which results in the transponder experiencing a relatively large amount of heating during the melt-sealing operation and produces long cycle times, i.e., low output of the melt-sealing machine. In addition, the coil only has an extremely limited life.
So-called reed glasses are also already known and are used to encapsulate reed switches and can be melted by means of infra-red radiation, which can be focused and is also effective in a vacuum. Infra-red melting has the advantage of the focusing which is possible enabling large amount of energy to be concentrated at points, so that the melting only takes a short time and thus the encapsulated components only experience a very small amount of heating. However, these reed glasses are not suitable for encapsulation of implantable transponders since they contain a high proportion of toxic constituents (B.sub.2 O.sub.3, BaO, PbO, etc.) and are therefore harmful in the long term.